Fix race condition between log_.erase and log_.back

Summary:
log_ contract specifies that it should not be modified unless both mutex_ and log_write_mutex_ are held. log_.erase however does that with only holding mutex_. This causes a race condition with two_write_queues since logs_.back is read with holding only log_write_mutex_ (which is correct according to logs_ contract) but logs_.erase is called concurrently. This is probably the cause of logs_.back returning nullptr in https://github.com/facebook/rocksdb/issues/3852 although I could not reproduce it.
Fixes https://github.com/facebook/rocksdb/issues/3852
Closes https://github.com/facebook/rocksdb/pull/3859

Differential Revision: D8026103

Pulled By: maysamyabandeh

fbshipit-source-id: ee394e00fe4aa520d884c5ef87981e9d6b5ccb28

db/db_impl.cc

@@ -810,6 +810,8 @@ void DBImpl::MarkLogsSynced(uint64_t up_to, bool synced_dir,
     assert(log.getting_synced);
     if (status.ok() && logs_.size() > 1) {
       logs_to_free_.push_back(log.ReleaseWriter());
+      // To modify logs_ both mutex_ and log_write_mutex_ must be held
+      InstrumentedMutexLock l(&log_write_mutex_);
       it = logs_.erase(it);
     } else {
       log.getting_synced = false;